Machine tool hydraulic system



Nov. 30, 1965 A. H. DALL MACHINE TOOL HYDRAULIC SYSTEM 4 Sheets-Sheet 1Filed Aug. 28, 1963 INVENTOR. ALBERT H. DALL Fig.1

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ATTORNEYS NOV. 30, 1965 L MACHINE TOOL HYDRAULIC SYSTEM 4 Sheets-Sheet 2Filed Aug. 28, 1963 Nov. 30, 1965 Filed Aug. 28, 1963 MACHINE TOOLHYDRAULIC SYSTEM 4 Sheets-Sheet 4 Fig.5

meal uazmfpv use/meg United States Patent 3,220,312 MACHINE TOOLHYDRAULIC SYSTEM Albert H. Dal], Cincinnati, Ohio, assignor to TheCincinnati Milling Machine Co., Cincinnati, Ohio, a corporation of OhioFiled Aug. 28, 1963, Ser. No. 305,083 8 Claims. (Cl. 90-11) The presentinvention relates to a hydraulic system in a machine, particularlysuitable for the drive of a member, such as a machine tool cutter, whichmay be subjected to a suddenly increasing load.

When the load on a hydraulically driven machine tool member, such as aslide or rotating tool, increases, the load on the hydraulic motor whichdrives the member increases accordingly. With this increased load, thepressure of the hydraulic fluid supplied through a pressure line by apump to the inlet side of the motor increases, and the fluid undergoes aslight compression. To provide continued operation of the motor withouta drop in speed, the pump must not only supply the fluid required tokeep the motor operating at the desired speed, but, because of thepressure increase resulting from the increased load, must also supplyadditional fluid to make up for the compression of fluid in the pressureline.

If the increase of load on the motor is gradual, the compression offluid in the pressure line is gradual and, under these conditions, thepump can usually supply fluid at a rate to compensate for the gradualcompression of fluid in the pressure line while, at the same time,supplying suflicient fluid at the rate required to keep the motoroperating without a significant drop in speed. However, a suddenincrease of the load on the motor results in a sudden compression of thefluid in the pressure line, and fluid for continued operation of themotor is not available until the pump has had time to supply theadditional fluid in the pressure line required because of thecompression of the fluid therein.

In some instances, particularly where the pressure line between the pumpand the motor is relatively long, the momentary hesitation resultingfrom the application of a sudden load can have serious consequences. Forexample, in a large milling machine, where a cutter mounted on acarriage is rotated by a hydraulic motor mounted on the carriage remotefrom the pump in the base, a sudden increase in the load on the motordue, for example, to full sudden engagement of the cutter with aworkpiece, will cause the motor to hesitate until the long pressure linebetween the pump and the motor can be filled by the pump as the fluid inthe line compresses under the increased pressure therein caused by theincreased load. However, during the time required for the pump to supplythe fluid necessary to compensate for compression in the pressure linefluid, relative feed movement between the carrier and workpiececontinues, increasing the load on the motor. This can cause a completestalling of the cutter and fracture thereof as the feed movementcontinues.

One solution to this problem would be the installation of a largeflywheel on the cutter spindle. With the cutter spindle rotating at thedesired speed before the cutter is subjected to a sudden increase inload, the flywheel would define a source of stored kinetic energy whichwould be instantly available to keep the cutter spindle and cutterrotating while the pump is supplying fluid to the pressure line tocompensate for the compression of fluid therein. However, the weight ofthe flywheel increases wear on the spindle bearings, and the increasedinertia of the spindle cutter slows down starting and stopping of thecutter. Moreover, one advantage in using a hydraulic motor on a toolcarriage to drive the tool instead of an 3,22%,3l2 Patented Nov. 30,1965 electric motor lies in its lighter weight, which is more easilysupported by the carriage, and the use of a flywheel on the spindlecutter would substantially diminish this advantage. Finally, the use ofa flywheel is applicable only with rotary driven members.

Another possible solution would be to increase the capacity of the pumpsupplying the hydraulic motor, or provide additional pump capacityavailable to the hydraulic motor, so that the demands of the hydraulicmotor can be met at the same time the pump is supplying additional fluidto the pressure line to make up for the compression of the fluid thereinwhen the load on the cutter increases sharply. Despite the fact thatonly a relatively small amount of fluid is required in the pressure lineto compensate for compression of fluid therein, there is, in the case ofa cutter being continuously fed into the work, only a very short timeavailable to supply this fluid, and any additional pump capacityprovided to meet this infrequent demand would have to be impracticallylarge to be capable of supplying fluid at the high rate needed.

In the present invention there is provided a hydraulic system which,like the flywheel, provides a source of energy instantly available whena sudden load in encountered, but which offers many advantages over theflywheel. With the present invention, fluid can be supplied to thepressure line leading to the inlet side of the motor when needed at ahigh rate without the expense of large pump capacity. In brief, anauxiliary source of stored fluid under high pressure is provided for thehydraulic motor. This source is normally isolated from the motor, and isconnected to the inlet side of the motor only when a sudden increase inload on the motor is encountered. Since only a small amount of fluid isrequired at a high rate, the auxiliary source of fluid under pressure isconnected only briefly to the inlet side of the motor and whendisconnected therefrom, is recharged. It is important, in the system ofthe present invention, that the auxiliary source of fluid under pressurebe normally isolated from the pressure line leading to the inlet side ofthe motor. If such a source were continuously connected to this line, itwould take fluid from the line when the pressure in the line was highand deliver fluid to the line when the pressure in the line was low. Toprevent stalling of a hydraulic motor encountering a sudden load inaccordance with the present invention, however, it is necessary thatfluid be supplied at a high rate to the line leading to the inlet sideof the motor when the pressure in that line is rising.

In the preferred form of the invention, the auxiliary source of storedhydraulic fluid under pressure comprises a hydraulic pressureaccumulator which may be conveniently located in the base ofthe machine,a blocking valve which is preferably located near the motor, and a lineconnecting the accumulator to the blocking valve. The discharge linefrom the motor contains a restriction to establish a back pressure inthe discharge line between the motor and the restriction which varies asthe speed of the motor varies. The blocking valve has an operating portconnected to the discharge line between the motor and the restriction,and has a discharge port connected to the inlet side of the motor. Theinstant the member, such as a rotary tool, driven by the hydraulic motorencounters a load sufficiently sudden to slow the hydraulic motorsignificantly, the back pressure drops to operate the blocking valve forthe release of a surge of fluid under high pressure from the auxiliarysource to the inlet side of the motor. This surge of fluid compensatesfor compression of the fluid in the line between the pump and the inletside of the motor so that the motor can instantly resume normal speeddespite the increase in load on the motor. As soon as the motor resumesspeed, the accumulator is again isolated from the motor.

Unlike the mechanical flywheel, this system does not increase wear ofthe spindle bearings, does not prolong starting and stopping of thespindle, and can be utilized to overcome sudden large loads applied to aslide as well as those applied to a rotary member.

In a machine tool having a pump in the base, and having a movablecarriage with a hydraulic drive motor thereon to drive, for example, arotary tool mounted on the carriage, the accumulator, which isrelatively heavy, is preferably mounted in the base, and the blockingvalve is preferably mounted on the carriage. With this construction, themajor portion of the weight of the system utilized to provide theauxiliary source of stored fluid under pressure is kept ofi thecarriage, but, at the same time, the high pressure stored fluid isavailable right at the motor on the carriage where it is needed withouttravel through a long line which inevitably produces resistance to flowwith a resultant pressure drop.

It is therefore one object of the present invention to provide ahydraulic system operable to prevent stalling of a hydraulic drive motorwhen subjected to a sudden load.

It is another object of the present invention to provide a source ofstored fluid under pressure at a hydraulic drive motor on a carriagewithout the necessity of mounting heavy components on the carriage.

It is still another object of the present invention to provide animproved hydraulic system for maintaining the speed of a hydraulic drivemotor.

It is yet another object of the present invention to provide a systemfor maintaining the speed of a hydraulic drive motor by providing asource of stored fluid under pressure which is released automatically tothe inlet side of the hydraulic motor when the speed of the motor drops.

It is a further object of the present invention to provide a hydraulicsystem operable to keep a cutter rotating as it feeds into a workpiecewithout significant loss of speed when resistance to the cutter suddenlyincreases.

Other objects and advantages of the present invention should be readilyapparent by reference to the following specification, considered inconjunction with the accompanying drawings forming a part thereof, andit is to be understood that any modifications may be made in the exactstructural details there shown and described, within the scope of theappended claims, without departing from or exceeding the spirit of theinvention.

In the drawings:

FIG. 1 is a fragmentary view in elevation, with parts broken away forclarity, of a milling machine having a hydraulic system constructed inaccordance with the present invention;

FIG. 2 is a schematic diagram of the hydraulic system shown in FIG. 1;

FIGS. 3 and 4 are graphs showing the forward pressure, back pressure,and speed of the hydraulic drive motor plotted against time before thesource of stored fluid under pressure is connected to the motor; and

FIG. 5 is a graph showing the relationship between rotation of thecutter and time for cutter speeds below, at, and above stalling speed.

There is shown in FIG. 1 part of a milling machine having a base onwhich an elongated table 11 is mounted for longitudinal movement. Twocolumns 12 (only one of which is shown) extend upwardly from the base,one on each side of the table, and support a bridge portion 13 extendingover the table. The bridge portion 13 has ways 14 on which a carriage 15is mounted for cross movement with respect to the table. Thelongitudinal movement of the table on the base and the cross movement ofthe carriage on the bridge are effected by conventional hydraulic powermeans (not shown) independent of the hydraulic system shown.

A cutting tool 16 is rotatably mounted on the carriage 15 and isconnected through a transmission 17 to a rotary hydraulic motor 18 alsomounted on the carriage. Relative feed movement between a workpiece 19mounted on the table and the cutting tool 16 is effected by movement ofthe table 11, or movement of the carriage 15, or both.

The hydraulic system for rotating the cutter 16 is shown in FIG. 1. Ahydraulic pump 20, and an electric motor 21 to drive the pump, aremounted in the base. The pump takes fluid from a sump 22 and delivers itunder pressure to a pressure line 23. A relief valve 24, connected toline 23 and discharging to the sump, is provided to prevent overload andrupture of the pressure line. Pressure line 23 is connected to the inletport 25 of the cutter motor 18 and, since the pump 20 is located in thebase and the cutter motor is located on the carriage, the line 23 mustbe relatively long. The return, or discharge, line from the outlet port26 of the motor to the sump contains a throttle valve 27 which defines arestriction adjustable in value by adjustment of throttle valve handle28 to rotate the valve core 27a connected thereto. The throttle valve,which may be secured in any convenient location, divides the return lineinto two sections: a secion 29a extending between the motor 18 and thethrottle valve and a section 29a extending between the throttle valveand the sump. The pressure in return line section 2%, because it is incontinous communication with the sump, will always be approximatelyatmospheric pressure. The pressure in return line section 29a will be atsome higher value whenever the motor is running and thereforedischarging hydraulic fluid. The pressure in return line section 2% willvary in accordance with the rate of discharge of fluid from the motor(and hence will vary in accordance with the speed of rotation of themotor and the speed of rotation of the cutting tool).

A hydraulic pressure accumulator 30 is mounted in the base and isconnected by line 31 to a blocking valve 32 mounted on the carriageclose to the hydraulic motor 18. A small, high pressure, hydraulic pump33, driven by electric motor 34, takes fluid from sump 22 and deliversit under pressure to line 31. A relief valve 35, connected on the outletside of the pump and discharging to the sump, establishes the maximumpressure in line 31. The blocking valve 32 has an inlet port 36connected to line 31, and an outlet, or discharge, port 37 connectedthrough the relatively short line 38 to the inlet port 25 of thehydraulic motor 18. The blocking valve 32 also has an operating port 39connected by line 40 to return line section 29a and operates in responseto the pressure in line section 29a, opening to connect the accumulator30 to inlet port 25 of the motor when the pressure is below apredetermined value and closing to isolate the accumulator from themotor when the pressure is above that predetermined value.

During a normal cutting operation (assuming a constant feed rate betweenthe rotating cutter and the workpiece with a normal cutter toothpenetration into the workpiece of, say, 0.13 inch) the forward pressure3 on the motor (that is, the pressure in line 23) will be constant understeady state conditions (that is, when there is no variation of the loaddefined by the engagement of the cutter with the workpiece) and will beat a value of, say, 2000 pounds per square inch. The relief valve 24,which is provided merely to prevent rupture in the system, is set tooperate at a relatively high pressure of, say, 5000 pounds per squareinch. Under normal cutting conditions the back pressure 11 on the motor(that is, the pressure in line 29a) will be constant under steady stateconditions (that is, when there is no variation in the speed of themotor) and will be at a value of, say, pounds per square inch. Underthese conditions, blocking valve 32 will be closed to isolate theaccumulator 30 and line 31 from motor 18.

If there is a gradual change in load on the motor (because, for example,of a gradual increase in the feed rate and a deeper penetration of theteeth of the cutter in the workpiece) of the forward pressure 1 willrise gradually. This pressure rise compresses the fluid in line 23, butsince the pressure rise is gradual, the constant volume pump 20 cansupply the small amount of additional oil required to fill line 23without any significant drop in speed of the motor. Since the backpressure b is directly related to the speed of the motor, there is nosignificant drop in the back pressure and blocking valve 32 remainsclosed.

If, however, there is a sudden change in the load on the motor, theforward pressure will rise sharply and, in an instant, the fluid in theline 23 will be compressed. Since the pump cannot in this instant, bothsupply fluid to compensate for the sudden compression of the fluid and,at the same time, supply sufficient fluid to keep the motor operating ata constant speed, the motor will slow down and, consequently, the backpressure b will drop. This drop in back pressure will open the blockingvalve to connect the accumulator 30 to the motor.

Although the accumulator 30 may be of any suitable conventional type, abladder-type accumulator is illustrated. As shown in FIG. 2, theaccumulator has a bladder 30a dividing the accumulator into twochambers: a chamber 30b containing a fixed mass of air introducedthrough check valve 30c, and a chamber 30d in communication with line 31and containing hydraulic fluid. With the valve 32 closed and the pump 33supplying fluid, the air in chamber 30b is compressed to form areservoir of high pressure. The instant valve 32 is opened, thispressure produces a surge of fluid out discharge port 37 of the valve 32at a substantially higher rate than could be produced by pump 33. Thus,the accumulator 30, line 31, and valve 32 define an auxiliary source ofstored fluid under high pressure for the motor 18. This source iscontinuously charged by the small, high pressure, low volume pump 33.If, for example, relief valve 35 is set to open at a pressure of 5000pounds per square inch, this pressure will be available at the dischargeport 37 of the blocking valve 32 on the carriage despite the fact thatthe accumulator 30 is located in the base. When the blocking valve 32 isopened, a surge of fluid is communicated through the short line 38,without significant pressure drop, to the inlet port 25 of motor 18 toincrease the speed of the motor and the cutter. As the motor speedincreases, the back pressure b increases to close the blocking valve 32.After the blocking valve 32 is closed, any loss of pressure in line 31due to the surge of fluid delivered to the motor is made up by pump 33.

For a more complete understanding of the operation of the presentinvention, reference is made to the schematic diagram of FIG. 2. Forconvenience, the effect of compression of the hydraulic fluid in line 23before value 32 is opened can be considered separately from the etlectof the fluid on the motor 18. In the diagram of FIG. 2, the line 23between the pump 20 and the motor 18 is represented by a container 23aof a volume equal to the volume of line 23 between the pump and themotor in which all compression of the fluid between the pump and themotor occurs, and by a line 23b of zero volume in which all fluidavailable to the motor flows. In other words, the flow rate, or output,Q of the pump 20 is made up of a flow q (into container 23a) whichmerely compensates for compression of fluid in line 23 and a flow q (inline 23b) which is available to the motor. At all times Q=q +q The flowq, in cubic inches per second equals (V/E) ,times (df/dt) where V is thevolume of line 23 between the pump and the motor in cubic inches, E isthe bulk modulus of the fluid in pounds per square inch, and df/a't isthe rate of increase of the forward pressure 1 in the 5 system (beforevalve 32 is opened) in pounds per square inch per second.

If d is the motor displacement in cubic inches per revolution, the motordisplacement in cubic inches per radian may be expressed as d/21r. Ifd0(m)/dt is the speed of the motor in radians per second, the flow q tothe motor must equal (d/27r) (dem/dt) in cubic inches per second. ThusThus, with a pump of constant volume output Q, the speed of the motorwill equal Q(Z1r/d) for any given steady state period when the pressurein the system is constant. However, the speed of the motor will bediminished in any given period (before valve 32 is opened) when thepressure is rising. The greater the rate of pressure increase, theslower the speed of the motor. This relationship is shown best in FIGS.3 and 4. FIG. 3 shows how a given rise A in the forward pressureoccurring in time At, causes a drop Ad6(m)/dt in the speed of the motor.FIG. 4 shows that the same rise A in the forward pressure 1, ifoccurring in a shorter time interval At, will cause a larger dropAd6(m)/dt in the speed of the motor.

Since the restriction defined by throttle valve 27, although adjustable,does not vary during operation, the back pressure b (that is, thepressure between the outlet side of the motor and the restriction) willvary in accordance with the speed of the motor, as indicated FIGS. 3 and4. It is this back pressure which is in communication with the operatingport 39 of the blocking valve 32. As shown in FIG. 2, the operating port39 of the valve is in continuous communication with one end of valveplunger 41 of blocking valve 32 to urge the valve plunger to the left(as viewed in FIG. 2) in opposition to a spring 42 in the valve, andclosing the valve when the back pressure rises above some predeterminedvalue. The valve member 41 shifts to the right as the back pressuredrops and, when the back pressure drops below the predetermined value,the valve opens, connecting the accumulator line 31 to the inlet side ofthe motor. The spring is adjustable by screw 43 so that the pressure atwhich the valve opens and closes can be selected.

The speed of the motor d0 (m) dt before valve 32 is opened, where R isthe ratio of the gear train in the transmission and d0(c)/dt is thespeed of the cutter so that Integrating this expression yields, when thelimits of integration are at t=0 and (f+Af) at t=t where t is time inseconds and A1 is the increase of pressure in time t.

When a workpiece is fed into a rotating cutter (or a rotating cutterinto a workpiece) each tooth, in traversing the span 21r/n radiansbetween adjacent teeth (where n is the number of teeth on the cutter),will penetrate the workpiece some distance depending on the relationbetween the feed rate and the rate of rotation of the cutter. Assuming aconstant feed rate, the penetration of the teeth of the cutter on eachrotation of 21r/rr radians will vary inversely with the speed of thecutter motor. In other words, the slower the cutter motor rotates, thedeeper the penetration of the cutter teeth into the work, and thegreater the load on, or resistance to continued rotation of, the motor.

010(0) equals Ii3 If, in a normal cut with a constant feed rate betweenthe tool and the work, each tooth of the tool penetrates the work adistance x equal to .013 inch (as shown in FIG. 2) as it traverses thespan 21r/n radians, the resistance to cutter movement will be greatenough to stall the cutter motor (if no source of auxiliary fluid underpressure were available) at some greater tooth penetration, say, 1.5x(or .0195 inch). It is therefore necessary for each tooth to complete arotation of 21/11 radians in order to encounter the cut of the precedingtooth) before a relative feed movement of 1.5x occurs between the workand the cutter. This relationship is shown in FIG. 5, assuming aconstant feed rate. Each line shown indicates the relationship betweenangular rotation of the cutter and time so that, in each case, the slopeof the line represents a cutter speed. FIG. 5 shows that at cutterspeeds below a critical cutter speed, each tooth will fail to reach thecut of the preceding tooth before a feed movement of 1.5x distanceoccurs so that, at these speeds, the cutter and cutter motor will stall.Since cutter speed is proportional to motor speed and motor speed isproportional to back pressure, the back pressure provides a reliablesignal for release of the auxiliary supply of stored fluid underpressure.

Screw 43 of blocking valve 32 is set so that the valve opens (that is,the valve plunger 41 moves to the right of the position shown in FIG.2), connecting the accumulator to the inlet side of the motor, when theback pressure (which is determined by the cutter motor speed) drops to avalue, say 100 pounds per square inch, in response to a drop in cuttermotor speed which drops the cutter speed to a value approaching thecritical cutter speed. The opening of valve 32 introduces high pressurefluid to the forward pressure line 23 and raises the speed of the cuttermotor before it stalls. As the cutter mot-or speed increases, the backpressure increases, and the blocking valve member 41 is shifted to theleft (to the position shown in FIG. 2) to again isolate the accumulator30 from the cutter motor. While the accumulator is isolated from thecutter motor, it is recharged by the highpressure, low volume, pump 33.

If the power available at the pump motor 21 is incapable of rotating thecutter motor (even if these is no compression of oil in the system) whenthe teeth of the cutter engaged with the work penetrate .0195 inch ordeeper into the work, and if, for example, the relative feed ratebetween the work and the cutter is inches per minute, or 0.17 inch persecond, a stalling penetration of .0195 inch will be realized in.0195/.167 or .117 second. If it be assumed that the cutter has 12teeth, the cutter must rotate one tooth span, or 21r/ 12 radians in .117second or less to prevent stalling. Therefore, the critical speed of thecutter is 21r/12(.117), or 4.47 radians per second.

It has been shown that the speed of cutter may be expressed as d6(.c)Mr: 2 /Rd) (Q) (V/E) (Zr/Rd) (df/dt) Assume, for illustrative purposes,that the ratio R of motor rotation to cutter rotation is 17.5, thatmotor displacement d is 4.07 cubic inches per revolution, that theconstant output of the pump Q is 76 cubic inches per second, that thevolume of the line between the pump and the motor is 300 cubic inches,and that the bulk modulus E of the oil utilized as the hydraulic fluidis 238,000 pounds per square inch. With no change of pressure, and henceno compression of oil in line 23, the speed of the cutter will be 6.70radians per second. The rate of change of pressure df/dt at the criticalcutter velocity of 4.47 radians per second, however, is 20,100 poundsper square inch per second, producing a change of pressure M of 20,100(.117), or 2350 pounds per square inch in the time required for stallingpenetration. The change in volume AV of the oil is equal to AfV/E thatis, (2350) (300)/(238,000), or 2.96 cubic inches of fluid. To replacethis fluid in .117 second or less requires a minimum flow of 2.96/.117,or 25.3 cubic inches per second, which the accumulator can easily supplysince only a small volume is required at infrequent intervals. It willbe noted that pump 33 can have a flow rate substantially below thisvalue, say 3.0 cubic inches per second, since it has ample time aftervalve 32 closes to recharge the accumulator.

What is claimed is:

1. In a machine tool having a rotary cutting tool,

(a) a hydraulic drive motor to rotate the tool,

(b) a hydraulic pump to supply fluid under pressure to the motor foroperation thereof,

(c) an accumulator normally isolated from the motor,

(d) and means responsive to a sudden increase in load on the cuttingtool to connect said accumulator momentarily to the motor.

2. In a machine tool having a rotary cutting tool,

(a) a rotary hydraulic drive motor to rotate the tool,

(b) a hydraulic pump to supply fluid under pressure to the inlet side ofthe motor for operation of the motor,

(0) an accumulator normally isolated from the inlet side of the motor,

(d) means responsive to a sudden increase in load on the cutting tool toconnect said accumulator momentarily to the inlet side of the motor,

(e) and means to charge the accumulator.

3. In a machine tool having a rotary cutting tool,

(a) a rotary hydraulic drive motor to rotate the tool,

(b) a first hydraulic pump to supply fluid under pressure to the inletside of the motor for operation of the motor,

(c) an accumulator normally isolated from the inlet side of the motor,

(d) a valve operable in response to the speed of the motor to connectthe accumulator to the inlet side of the motor when the speed of themotor drops below a predetermined value and to disconnect theaccumulator from the inlet side of the motor when the speed of the motorrises above said predetermined value,

(e) and a second pump to charge the accumulator.

4. In a machine tool having a rotary cutting tool,

(a) a hydraulic drive motor connected to said cutting tool to effectrotation thereof,

(b) a hydraulic pump connected to the inlet side of the motor foroperation thereof,

(c) a discharge line connected to the outlet side of the motor, saiddischarge line having a restriction therein to establish a back pressurein the discharge line between the motor and the restriction,

(d) a hydraulic accumulator normally isolated from the inlet side of themotor,

(e) and means to connect said accumulator to the inlet side of the motorin response to a drop in said back pressure.

5. In a machine tool having a rotatable cutting tool,

(a) a rotary hydraulic motor connected to said cutting tool for rotationthereof,

(b) a first hydraulic pump connected to the inlet side of the motor foroperation thereof,

(c) a discharge line connected to the outlet side of the motor, saiddischarge line having a restriction therein to establish a back pressurein the discharge line between the motor and the restriction varying asthe speed of the motor varies,

(d) a hydraulic accumulator normally isolated from the inlet side of themotor,

(e) means including a blocking valve operable in response to variationof said back pressure to isolate the accumulator from the inlet side ofthe motor when the back pressure is above a predetermined value and toconnect the accumulator to the inlet side of the motor when the backpressure drops below said predetermined value,

(f) and a second hydraulic pump to charge said ac- 8. In a machine toolhaving a base and having a carriage movable relative to the base, saidcarriage having a tool rotatably mounted thereon, the combinationcomprising:

cumulator. 6. In a machine tool having a rotatable cutting tool, (a) arotary hydraulic drive motor connectable to said cutting tool to effectrotation thereof, (b) a hydraulic pump, (c) a pressure line connectingthe pump to the motor, (d) a hydraulic accumulator, a blocking valve,and a line connecting the accumulator to the blocking valve to define asource of stored fluid under pressure, said blocking valve located closeto the motor,

(e) a line connected between the blocking valve and the inlet side ofthe motor,

(f) and means responsive to an increase in load transmitted to the motorthrough said cutting tool to operate the blocking valve and releasestored fluid under pressure to the inlet side of the motor.

7. In a machine tool having a base and having a carriage movablerelative to the base, said carriage having motor, said discharge linehaving a restriction therein to establish a back pressure in thedischarge line between the motor and the restriction, varying as thespeed of the motor varies,

(e) a source of stored fluid under pressure comprising a hydraulicaccumulator mounted in the base, a blocking valve mounted on thecarriage, and a line connecting the blocking valve to the accumulator,

(1) said blocking valve having a discharge port a tool rotatably mountedthereon, the combination comconnected to the inlet Side of the hydraulicprising: motor and having an operating port connected to (a) a rotaryhydraulic motor supported by the carriage the discharge line between t mt r and the and connected to rotate the tool, restriction, (b)ahydraulic pump in the base, (2) said blocking valve operable inresponse to (c) a pressure line connecting the pump to the inlet thePressure at Said Operating P t0 Close Said side of the motor, dischargeport when the speed of said motor is (d) a discharge line connected tothe outlet side of the above a pr ermined Value and to open Said motor,said discharge line having a restriction therein discharge P when theSpeed of Said IIlOtOI' to establish a back pressure in the dischargeline ber p low said predetermined value, tween the motor and therestriction, (f) and a second hydraulic pump in the base connected (e) ahydraulic a u l to ou t d i th b d to said source of stored fluid underpressure to charge a blocking valve mounted on the carriage in com- Saidemunication with the accumulator to define a source of stored fluidunder pressure, said blocking valve References Cited by the Examine!having a discharge port connected to thet inlet side UNITED STATESPATENTS of the hydraulic motor and having an operating port connected tothe discharge line between the motor $152 2? et 72 5 and therestriction, said blocking valve operable in 2674850 4/1954 S r responseto the pressure at said operating port to Venson e a close saiddischarge port when said back pressure 40 WILLIAM DYER JR, PrimaryExaminer.

is above a predetermined value and to open said discharge port when saidback pressure drops below said predetermined value.

1. IN A MACHINE TOOL HAVING A ROTARY CUTTING TOOL, (A) A HYDAULIC DRIVEMOTOR TO ROTATE THE TOOL, (B) A HYDRAULIC PUMP TO SUPPLY FLUID UNDERPRESSURE TO THE MOTOR FOR OPERATION THEREOF, (C) AN ACCUMULATOR NORMALLYISOLATED FROM THE MOTOR, (D) AND MEANS RESPONSIVE TO A SUDDEN INCREASEIN LOAD ON THE CUTTING TOOL TO CONNECT SAID ACCUMULATOR MOMENTARILY TOTHE MOTOR.